The present invention relates generally to turbo-generators, and more particularly to bearing support and control systems for permanent magnet generator based turbo-generators.
In airborne applications that require the generation of both cool and conditioned pressurized air as well as the generation of electric power, the use of a turbo-generator driven by engine bleed air provides a compact, contained system that does not adversely affect the performance of the aircraft as compared to, e.g., other generator systems. In a turbo-generator, a shaft is rotatably supported in a housing and serves to connect a permanent magnet generator and a turbine. The pressurized supply air is supplied to and expanded in the turbine where it achieves a very low temperature in order to provide cooling. The compressed air acting on the turbine rotates the shaft which, in turn, drives the permanent magnet generator (PMG). The rotation of the PMG rotor generates a rotating magnetic field in the stator windings. This produces an electric power output whose voltage and frequency are directly related to the speed of rotation of the rotor.
To support the shaft which connects the PMG and the turbine, a turbo-generator typically employs three bearings. Two of the bearings are radial bearings which prevent the shaft from shifting radially. The third bearing is a thrust bearing which holds the shaft in a fixed axial position. If the bearings permit more than just slight amounts of free play, the shaft will shift under influence of the strong permanent magnets of the generator rotor or when the turbine is loaded and will allow the rotor or the blade tips to contact the encircling housing.
In prior turbo-generators, bearings, e.g. ball bearings, hydrodynamic fluid film bearings (commonly called air bearings), etc., have been used to locate the shaft radially and axially since such bearings provide minimal free play. Air bearings, however, require extremely small clearances, require supply air for cooling, and are slightly damaged at each start up and shut down as a result of lack of support at low speeds. Because of the small clearance in the air bearings, dirt or other combination increases the susceptibility for damage and, in addition, such clearances result in relatively high friction and heat. As a result, bearing replacement is a frequent cost item and, if failure occurs during operation, the PMG and/or turbine may be damaged. A shaft which is supported by air bearings must be removed axially from the housing for repair or replacement and this usually requires that the turbo-generator be removed from the aircraft and sent to a remote repair depot.
Magnetic bearings are used for supporting shafts in various types of machinery. In radial magnetic bearings, several electromagnets are spaced angularly around a shaft and, when energized, produce opposing magnetic forces which cause the shaft to levitate in free space within the housing. Sensors detect the actual position of the shaft and vary the energization of the electromagnets in such a manner as to keep the shaft centered precisely on a predetermined axis. One such system is described in U.S. Pat. No. 5,310,311, entitled Air Cycle Machine With Magnetic Bearings, assigned to the assignee of the present application, the teachings and disclosure of which are hereby incorporated in their entireties by reference thereto. While the initial cost of a magnetic bearing system may be somewhat higher than that of an air bearing system, magnetic bearings permit more easily attainable machining tolerances and larger clearances, require no air for cooling, experience a relatively long service life, and are capable of supporting shafts either at rest or operating at speeds of 100,000 RPM or higher.
Despite the clear advantages provided by magnetic bearings, their use has been precluded for PMG-based turbo-generators designed for airborne operation. In these applications, and particularly in fighter aircraft operation, the turbo-generator assembly is highly compact and operates at high rotational speeds in an effort to reduce size and weight. Further, the permanent magnets used in the PMG have a high magnetic density to enable the maximum electrical output generation during operation with minimal weight. As a result of these factors, the environment within the turbo-generator contains a high degree of magnetic flux. Because the control of magnetic bearings relies on precise magnetic flux variation to correct the smallest shaft position variation, use of these devices in the highly magnetic environment of a PMG-based turbo-generator has been precluded. Such is particularly true for fighter aircraft applications in which extremely high dynamic forces caused by aircraft maneuvers are typical.
In view of the above, it is an object of the invention to provide a new and improved turbo-generator machine which utilizes magnetic bearings to precisely support the PMG/turbine shaft for rotation at high speeds in a high vibration, high shock, and high temperature environment such as typically exists in a jet aircraft.
A further object of the invention is to provide a turbo-generator having magnetic bearings and a housing which are uniquely assembled as a clamshell structure permitting relatively quick and easy removal of the shaft from the bearings and the housing for purposes of repairing or replacing the shaft and/or other components of the turbo-generator.
The invention also resides in the use of magnetic force for biasing the shaft radially in opposition to the side forces created by the magnetic flux acting between the permanent magnets and the stator of the PMG. This is aided by the coordination of design parameters such as the ratio between the magnetic bearing air gap and the PMG rotor/stator air gap. Further, precise control of the magnetic bearings is enabled by minimizing the axial magnetic leakage flux from the PMG that is allowed to disrupt the magnetic bearing control and sensing. In one embodiment this is accomplished by controlling the ratio between axial and radial cross section of the PMG stator and the shaft. Further, the system of the present invention allows the magnetic center of the PMG to float within the stator housing, i.e. no mechanical centering is required.
In one embodiment the turbo-generator is a radial inflow turbine with a permanent magnet generator. The turbine and generator are mounted on a single shaft that is supported by magnetic bearings. The turbo-generator includes rotor containment for the high-energy rotors. The magnetic bearing center section uses an axial split feature to allow replacement of the shaft and wheel assembly using common hand tools, thereby providing for high maintainability. This dramatically reduces overhaul time if service is required. The turbo-generator may be used in an exemplary system that provides both cold air for a radar poly alpha olephin (PAO) cooling loop and electrical power for the radar. The system is self-contained except for the bleed air connection, a start-up/shut-down electric power connection and an air exhaust.
In one embodiment of the present invention, a turbo-generator for an aircraft comprises a housing and a shaft disposed in the housing and having a central axis. A permanent magnet generator is mounted on the shaft. Further, a turbine mounted on the shaft in axially spaced relation with the permanent magnet generator. This turbine includes an inlet for receiving gas from an aircraft engine, and is driven by the gas to drive the permanent magnet generator via the shaft. The gas received by the turbine expands therein, is cooled as a result of expanding, and is exhausted as chilled gas by the turbine. Axially spaced bearings for radially supporting the shaft for rotation in the housing are also included. Each of the bearings comprises a number of electromagnets mounted within the housing and spaced angularly around the shaft. The electromagnets include selectively energizable electrical coils for producing magnetic forces to suspend the shaft radially within the housing. Radial position sensors for sensing the radial position of the axis of the shaft are also included. A magnetic bearing controller is responsive to the radial position sensors for varying the energization of the coils to keep the axis of the shaft in coincidence with a predetermined axis. An additional bearing for maintaining the shaft in a predetermined axial position in the housing is also included. This additional bearing comprises axially spaced and axially opposing electromagnets mounted within the housing and positioned substantially coaxial with the shaft. A disc that is rigid with and projects radially from the shaft is located between the axially opposing electromagnets. The axially opposing electromagnets also comprise selectively energizable electrical coils for producing magnetic forces acting in axially opposing relation on the disc. An axial position sensor for detecting the axial position of the shaft is also included. Preferably, the magnetic bearing controller is responsive to the axial position sensor for varying the energization of the coils of the axially opposing electromagnets to keep the shaft in a predetermined axial position.
In an alternate embodiment, radial backup bearings are provided to support the shaft radially when the radial magnetic bearings are not energized. In a further embodiment, axial backup bearings are also provided to support the shaft axially when the axial magnetic bearings are not energized. Preferably, the axial backup bearings are provided integrally with the axial magnetic bearings. In one embodiment, the axial backup bearings are provided by the face of the axial magnetic bearings on which the coils are wound.
In one embodiment the permanent magnet generator includes a permanent magnet rotor mounted on the shaft and a wound stator radially displaced therefrom by a first air gap distance. The bearings for radially supporting the shaft are radially displaced from the shaft by a second air gap distance, which is smaller than the first air gap distance. Preferably, the ratio of the second air gap distance to the first air gap distance ranges between approximately 1:3 and 1:4. In another embodiment the second air gap distance is approximately 0.015xe2x80x3, and the first air gap distance is approximately 0.050xe2x80x3. In a preferred embodiment, the second air gap distance is sized in relation to the first air gap distance such that the bearings resist side pull forces generated between the permanent magnet rotor and the wound stator.
In an alternate embodiment of the present invention, a magnetic flux cross section in an axial direction is small in relation to a magnetic flux cross section in a radial direction such that magnetic flux leakage from the permanent magnet generator to the bearings does not inhibit the bearings"" ability to maintain the axis of the shaft in coincidence with the predetermined axis. In one embodiment, the axial magnetic center of the permanent magnet rotor is allowed to float without mechanical bias thereby reducing thrust loading on the shaft.
In a further embodiment of the invention, the energization of the bearings for radially supporting the shaft is supplied by the magnetic bearing controller from power generated by the permanent magnet generator during operation thereof. Preferably, the energization of the bearings for radially supporting the shaft is supplied by the magnetic bearing controller from aircraft power until the turbine reaches a predetermined minimum speed. In one embodiment, the bearings for radially supporting the shaft comprise four quarter-circular segments positioned angularly around the shaft. In yet a further embodiment of the invention, the energization of the bearings for axially supporting the shaft is supplied by the magnetic bearing controller from power generated by the permanent magnet generator during operation thereof. Preferably, the energization of the bearings for axially supporting the shaft is supplied by the magnetic bearing controller from aircraft power until the turbine reaches a predetermined minimum speed. In an embodiment of the present invention that utilizes magnetic bearings for both radial and axial support of the shaft, the energization of the bearings for both radially and axially supporting the shaft is supplied by the magnetic bearing controller from power generated by the permanent magnet generator during operation thereof. Preferably, the energization of these bearings is supplied by the magnetic bearing controller from aircraft power until the turbine reaches a predetermined minimum speed
Other objectives and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.